Oximetry is the measurement of the oxygen status of blood. Early detection of low blood oxygen is critical in the medical field, for example in critical care and surgical applications, because an insufficient supply of oxygen can result in brain damage and death in a matter of minutes. Pulse oximetry is a widely accepted noninvasive procedure for measuring the oxygen saturation level of arterial blood, an indicator of oxygen supply. A pulse oximetry system consists of a sensor attached to a patient, a monitor, and a cable connecting the sensor and monitor. Conventionally, pulse oximetry sensors include a light source(s) characterized by a plurality of wavelengths of known spectra transmitting optical radiation into or reflecting off a measurement site, such as, body tissue carrying pulsing blood and/or interstitial fluid. After attenuation by tissue and fluids of the measurement site, a photodetection device(s) detects the attenuated light and outputs a detector signal(s) responsive to the detected attenuated light. A signal processing device(s) process the detector(s) signal(s) and outputs a measurement indicative of a blood constituent of interest, such as glucose, oxygen saturation, methemoglobin, carboxyhemoglobin, glycated hemoglobin, respiration rate, pulse rate, total hemoglobin, other physiological parameters, or other data or combinations of data useful in determining a state or trend of wellness of a patient.
The sensor is typically attached to a patient's finger or toe, or a very young patient's foot. For a finger, the sensor is configured so that the light source (for example, an LED) projects light through the fingernail and into the blood vessels and capillaries underneath. The photodiode is positioned at the fingertip opposite the fingernail so as to detect the transmitted light as it emerges from the finger tissues.